Valve actuation in an internal combustion engine is required in order for the engine to produce positive power and may also be used to provide engine braking. Typically, engine valves may be actuated in response to the rotation of cams. One or more lobes on the cam may displace the engine valve directly, or act on one or more valve train elements, such as a push tube or rocker arm, connecting the cam to the engine valve. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion and/or exhaust gas recirculation (EGR). The exhaust valves may be opened to allow combustion gas to escape from the cylinder and/or for EGR.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. This air compressor effect may be accomplished by cracking open one or more exhaust valves near piston top dead center position for compression-release type braking, or by maintaining one or more exhaust valves in a cracked open position for much or all of the piston motion for bleeder-type braking. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
For both positive power and engine braking applications, the timing of the opening and closing of the engine cylinder intake, exhaust, and auxiliary valves is determined by the shape or profile of cams with one or more fixed lobes. Fixed lobes on the cams may make it difficult to adjust the timings and/or amounts of engine valve lift needed to optimize valve openings and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic or other linkage means. Many lost motion systems use a hydraulic link to provide varying levels of valve actuation. Some hydraulic valve actuation systems that are adapted to selectively vary the amount of lost motion during engine operation are referred to as Variable Valve Actuation (VVA) systems.
An example of a variable valve actuation lost motion system is described fully in U.S. Pat. No. 6,510,824 to Vorih, et a., (Jan. 23, 2003), which is hereby incorporated by reference. Other examples of such systems are provided in Vorih, et al., U.S. Pat. No. 5,829,397 (Nov. 3, 1998), Hu, U.S. Pat. No. 6,125,828 (Oct. 3, 2000), and Hu, U.S. Pat. No. 5,680,841 (Oct. 28, 1997), and which are incorporated herein by reference.
An engine must have some level of valve actuation to start and continue to run. Without valve actuation, fresh air cannot be introduced into, and exhaust gas cannot be removed from, the cylinders. Engines that incorporate hydraulic valve actuation systems may require an immediate and sustained supply of hydraulic fluid to operate the engine valves. Therefore, it is desirable to have a sufficient supply of hydraulic fluid available for the valve actuation systems at the time of starting an engine.
It is therefore an advantage of some, but not necessarily all, embodiments of the present invention to provide hydraulic fluid to a hydraulic valve actuation system at engine start-up. It is also an advantage of some, but not necessarily all, embodiments of the present invention to retain some amount of hydraulic fluid in a hydraulic valve actuation system after engine shut-off.
Additional advantages of various embodiments of the invention are set forth, in part, in the description that follows, and in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.